The conventional solution to the problem of detecting deep defects in electrically conductive materials in a nondestructive manner is the use of a low-frequency eddy-current method using induction coils for excitation and detection purposes. This method has a sensitivity which is strongly dependent on the exciter frequency and breaks down properly at low frequencies. An attempt is generally made to compensate for the weakness of the sensor by increasing the exciter current. In the prior art, no particular value has been placed on preventing inductive signals which can easily exceed the magnetic-field signals, in particular at relatively high frequencies (kHz range).
VDet=V(B)+V(δB/δt), where V(B)<V(δB/δt) for relatively high frequencies.
The inductive signals are very inconvenient for two reasons. Firstly, additional lift-off signals may result which are perceived as defect signals, and secondly it is no longer possible for quantitative material testing to be carried out in a nondestructive manner since the amplitude and phase of the detector voltage have been distorted.